Active inductors are often used to extend the bandwidth of bandwidth-limited circuits, such as amplifiers employed in high frequency communication systems. The bandwidth of an active inductor-loaded amplifier (or buffer) is generally a function of the total output capacitance and resistance, along with an inductance “L” which, for an active inductor, is in turn a function of a resistance “R”, parasitic capacitance, and transconductance of the transistor utilized to implement the active inductor.
A number of techniques have been proposed or suggested for tuning an active inductor utilizing a fixed resistance value such that a portion of the capacitive load is canceled (and the bandwidth is thereby extended). Process variations in the manufacturing of integrated circuits, temperature variations, and supply variations, however, often significantly affect the resulting resistance, capacitance, and transconductance of an active inductor circuit. Thus, the resulting available bandwidth is also subject to wide variations, despite any selection of resistance or capacitance values for fabrication.
U.S. patent application Ser. No. 10/929,843, entitled. “Three-Terminal, Tunable Active Inductor,” assigned to the assignee of the present invention and incorporated by reference herein, discloses an active inductor circuit that is capable of being tuned to achieve a desired bandwidth, over process, temperature and supply variations. Generally, one embodiment of the disclosed active inductor circuit controls the gate voltages applied to one or more PMOS transistors to provide a variable resistance and thereby extend the bandwidth. While the disclosed active inductor circuit can be continuously tuned to achieve a desired bandwidth, it has been found that the range of resistance control required for some applications requires prohibitively large PMOS devices.
A need therefore exists for an improved active inductor device that provides a tunable bandwidth using digital control. A further need exists for an active inductor device that provides a tunable bandwidth with improved linearity.